This invention relates to power amplifiers and amplifying methods, and more particularly to high-efficiency power amplifiers and related methods Power amplifiers are widely used in communication systems, for example in radiotelephone base stations and radiotelephones. In radiotelephone communication systems, power amplifiers typically amplify high frequency signals for transmission.
A major consideration in the design of power amplifiers is the efficiency thereof. High efficiency is generally desirable so as to reduce the amount of power that is dissipated as heat. Moreover, in many applications, such as in satellites and portable radiotelephones, the amount of power that is available may be limited. An increase in efficiency in the power amplifier therefore may be desirable, in order to allow an increase the operational time or capacity for the satellite or portable radiotelephone.
In U.S. Pat. No. 2,210,028 to Doherty (August 1940), an arrangement of two vacuum tube power amplifiers coupled by a single quarter wave line is described. A semiconductor version of the Doherty amplifier is described in a more recent U.S. Pat. No. 5,420,541 entitled "Microwave Doherty Amplifier", to Upton et al. In Proc. IRE, Vol. 23 No. 11 (1935), pages 1370-1392, entitled "High Power Outphasing Modulation", Chireix describes producing a transmitter giving a modulated amplitude output signal by combining two constant output amplitude amplifiers with a variable phase difference so that their outputs can be varied in relative phase from additive to subtractive. In U.S. Pat. Nos. 5,568,088; 5,574,967; 5,631,604; and 5,638,024 to coinventor Dent, all entitled "Waste Energy Control and Management in Power Amplifiers", various arrangements of coupled power amplifiers are disclosed in which a varying amplitude signal may be produced using constant amplitude power amplifiers. In applicant's 1964 graduate thesis project, an amplifier was built and reported in which the value of Vcc was selected to be either Vcc or 0.7 Vcc based on whether the desired output amplitude was greater or less than 0.7 Vcc. Finally, in the 1960s, many so-called "class-D" or pulse-width modulation amplifiers were proposed and manufactured.